Lord Ashcroft was a Tory donor who gave £8million to the party, helping them into power under the coalition in 2010.

But even that caused controversy due to the billionaire's non-dom status - which Lord Ashcroft claimed to have given up 10 years previous.

They were later cleared of breaking election rules over the businessman's £5.1million donations through his company Bearwood.

But the dispute over Lord Ashcroft's tax status and Mr Cameron's failure to offer him a high level ministerial position in the Coalition Government are at the centre of his Call Me Dave book, written with journalist Isabel Oakeshott.

The 69-year-old was only offered a junior role despite donating around millions to the Tories since the party's 1997 election defeat and having served as deputy chairman during Mr Cameron's period as leader in opposition.

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Lord Ashcroft admitted he is now a critic of the PM saying he is "more interested in holding the office than in using its power to achieve anything".

He said the book is "not about settling scores".

But he did reveal his anger over being offered just the role of junior whip in the Foreign Office.

He said: "After putting my neck on the line for nearly ten years — both as party treasurer under William Hague and as deputy chairman — and after ploughing some £8million into the party, I regarded this as a declinable offer.

"It would have been better had Cameron offered me nothing at all."

He said Cameron told him former deputy prime minister Nick Clegg had blocked Lord Ashcroft's appointment to a key position.

However, Mr Clegg denied this today: "No I don't recollect that at all. It wouldn't have been my role, and nor by the way would it have been David Cameron's role to block my suggestions about Liberal Democrat ministerial appointments in Whitehall."

The former Lib Dem leader added: "I am now used to Conservatives - they certainly did it for five years - using me as an alibi for awkward decisions that they have to face within their own party.

"I have no doubt that this just fell into that same category."

Ms Oakeshott rejected that the book was a "revenge job", pointing out that Lord Ashcroft could have chosen to release it in the run-up to the General Election.

She said: "That would have caused far more damage, he could easily have done that. Or indeed we could have published the book over party conference.

"David Cameron has said he is not going to be serving a third term as party leader. He is not going to go on and on."